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All of a sudden the truck acted like the engine shut off?

Tiny
NORTHPORTMAZDA
  • MEMBER
  • 1991 MAZDA B2600
  • 2.6L
  • 4 CYL
  • 4WD
  • MANUAL
  • 137,000 MILES
I have a the truck listed above b2600i model that has been running like a dream until recently. I was driving down the road and all of a sudden the truck acted like the engine shut off. It was idling but as soon as I would give it gas it would want to shut off and the engine did not have enough power to move the truck.

The issue I am having is that a idles high, 1,500 to 2,000 rpm's, and as soon as you give it gas it wants to shut off, though you can vary slowly give it a little bit of gas but eventually it will bog down and indicator.

If you shake the wiring harness underneath the intake manifold sometimes it will all of a sudden drop down to the normal 750 idle speed and you can give it all the throttle you want and drive it just fine. Then all of a sudden it'll jump back up to a high idle speed and if you give it throttle it'll want to indicator.

I have a 1,300 page mechanics repair manual for a 1992 b2600i and I have been going through it. I haven't checked the circuit board down near the feet of the passenger side (as I just found out about it a moment ago on this site) but I have checked for water in the fuel line, fuel filter, fuel pressure, every sensor that has anything to do with the control system and they all seem to be good. I pretty much determined it's the wiring harness under the intake manifold, which I'm not sure of because I do not understand how that can go bad.

The other strange thing is that while it is idling at the high speed and regular engine temperature you can disconnect (all at the same time) the throttle sensor, the solenoid that coolant runs in and out of by the throttle sensor, idle switch/sensor, water thermosensor, temperature sensor, intake air thermosensor, oxygen sensor, a give it gas and the engine keeps running. The only thing that seems to affect it is when you unplug the idle switch the RPM's will go up a little bit. When you plug it back in it drops back down a bit. But from my understanding if you unplug the throttle sensor as soon as the idle switch is released something should happen because the computer then does not know how much gas to give the engine with the throttle sensor disconnected.

So my best guess so far is that I need to replace the short wiring harness that is located under the intake manifold and connects the main harness to the fuel injectors, throttle sensor, etc.

What is this short wiring harness called and where can I find it/purchase it?

Any other suggestions would be great!
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Monday, March 4th, 2019 AT 8:46 AM

50 Replies

Tiny
SCGRANTURISMO
  • MECHANIC
  • 4,897 POSTS
Hello,

That is usually called the fuel injector harness. You might want to check on eBay or Amazon as they usually have OEM or equivalent parts for about half of what parts houses charge. Good luck and let us know how it turns out.

Thanks,
Alex
2CarPros
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Tuesday, March 5th, 2019 AT 5:59 AM
Tiny
NORTHPORTMAZDA
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If it is not the wiring harness (which I am not having very good luck locating one) what else do you think it could be?

One suggestion is the Idol Air Controller that the throttle sensor is connected to. It may need to be taken apart and cleaned as it can get gummed up over time from the coolant running through it. Do you think this could be causing the problem? I don't want to have to take this apart if I don't need to.
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Tuesday, March 5th, 2019 AT 10:36 AM
Tiny
SCGRANTURISMO
  • MECHANIC
  • 4,897 POSTS
Hello,

Yes, it very well could be. To be perfectly honest with you, when I read your first question, that is exactly what I thought it was. I have included some information for you in the diagrams down below. Please go through them and get back to us with what you find out.

Thanks,
Alex
2CarPros
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Tuesday, March 5th, 2019 AT 5:22 PM
Tiny
NORTHPORTMAZDA
  • MEMBER
  • 14 POSTS
I pulled the idle air control valve unit off, tore it apart and cleaned up everything really good with carburetor cleaner and even cleaned the throttle body up. Put it all back together, started it up and still the same problem. Either high constant idle or up and down. If you try to give it gas it wants to shut off.

I also tried to check to see if there were any check engine codes (though the check engine light has never come on) by trying to ground out the single green connection by the fuel filter and I could never get it to work. Not sure what I am doing wrong there. When you turn the key to the "On" position the ABS light goes out but the check engine, oil, and battery lights stay on until you start the truck.

I found a good used Idle Air Control valve for $150.00 shipped, think I should but it and put it on? I tried checking it with an electric meter but I'm not sure I did it right cause I got no numbers at all. I'm not good at the kinda stuff - lol. Still no luck finding a wiring harness either if that the problem.
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Tuesday, March 5th, 2019 AT 9:24 PM
Tiny
SCGRANTURISMO
  • MECHANIC
  • 4,897 POSTS
Hello again,

Okay, usually when a vehicle is "searching" at idle that means you have a vacuum leak somewhere. Please go to the link down below.

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

Go through the guide and get back to us with what you find out.

Thanks,
Alex
2CarPros
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Wednesday, March 6th, 2019 AT 10:13 PM
Tiny
NORTHPORTMAZDA
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Okay, so no vacuum leaks that I can tell. Also replaced the Idle Air Controller. It gets cold here so I use a Kat's Coolant Heater when I need to. I plugged it in for a bit (about 30-45 minutes) and came back, checked that the coolant lines felt warm and started the truck. Some, but not much improvement.

Sense the engine was semi warm at start up from the coolant heater it idled at about 1,200 rpm for about two to three seconds and dropped down. Now it actually idles smoothly at 750 rpm, where it's supposed to, and doesn't surge up and down but it still wants to bog down or wants to shut off when you give it gas. Having a friend help me, while he was watching the tachometer and I was at the motor manually operating the throttle, I slowly gave it gas until it wanted to act up. Apparently at about 2,900 rpm is where he said it started to have issues. If you hold the throttle above 2,900 the rpm it runs really rough trying to fight to stay alive, though it still runs. If you hold it long enough it will try to die but at the last second come back, kinda.

I thought about trying to drive it sense I could get a tiny bit of throttle out of it, unlike before but didn't get a chance to.

Could it be the ECM or the injector wiring harness? I plan on taking off the cover to the ECM and see if there would be a corrosion issue I have read about happening.

Not sure if this helps but I have never had the check engine light come at all either. One friend said that it could be the crankshaft sensor, but on the 2600i it's actually part of the distributor.

Not sure what to do now.
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Sunday, March 10th, 2019 AT 1:18 AM
Tiny
NORTHPORTMAZDA
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Could it also be the throttle position sensor?
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Sunday, March 10th, 2019 AT 1:19 AM
Tiny
NORTHPORTMAZDA
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I ran the engine until the running temperature was normal then I disconnected the throttle position sensor while engine was running and gave it gas, it didn't make a difference one way or the other. Same thing when I unplugged everything else I could under the hood one at a time and gave it gas, it would climb to 2,900 and then have issues, with the exception of the idle sensor. When I unplugged it the rpm's went up a bit and it went back to normal (750) when I plugged it back in. The only electrical connectors I didn't unplug we're the mass air flow sensor and the distributor.
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Sunday, March 10th, 2019 AT 1:34 AM
Tiny
SCGRANTURISMO
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Hello again,

Glad to hear that it was the Idle Air Control valve (IAC) that fixed the "searching" idle. The Throttle Position Sensor (TPS) could be your mid rpm engine's problem. You might also want to check your fuel filter and the gap on your spark plugs as well.

Thanks,
Alex
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Sunday, March 10th, 2019 AT 3:20 AM
Tiny
NORTHPORTMAZDA
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  • 14 POSTS
Spark plugs are platinum plugs that said not to gap and have less than 3,000 miles on them. Fuel filter has less than 3,000 thousand miles on it but I could get a new one just in case. I drained it about a week ago and do a jar to see if there is any water in the fuel, and did a fuel pressure test and everything checked out.

I'll try to test the throttle position sensor again with an ohmmeter and see if I can pick up one and go from there.
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Sunday, March 10th, 2019 AT 7:23 AM
Tiny
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Hello again,

You might also want to check the Exhaust Gas Recirculation (EGR) system. I have included in the diagrams below Troubleshooting Diagnostic Step by Step lists of what to check for your vehicle from the manufacturer. Please go through the list as it is the definitve list from the manufacturer, and is guarenteed to fix your problem. All the tests require no special tools, are inexpensive, and can easily be done. Let us know what you find out.

Thanks,
Alex
2CarPros
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Sunday, March 10th, 2019 AT 2:54 PM
Tiny
NORTHPORTMAZDA
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I ordered a Throttle Position Sensor off of eBay today for $50.00 delivered, I'll have it Thursday afternoon.

I will go through the EGR stuff that you sent and check that out as well. I also have the mechanics 1,400 page manual for the truck so I will go through both.

Hopefully between the two it'll take care of the problem. I'll let you know what happens.
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Sunday, March 10th, 2019 AT 3:03 PM
Tiny
NORTHPORTMAZDA
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My truck doesn't seem to have an EGR system. That was for trucks made for California.

Any other suggestions while I wait for the throttle sensor. I got it because if it's something else I can return it. If it is then I got it. Someone else suggested looking at the mass airflow sensor. If the wire get dirty it can cause it to act up as well.
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Monday, March 11th, 2019 AT 9:11 AM
Tiny
SCGRANTURISMO
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Hello again,

Sure. It can cause all kinds of driveability issues. If you want to clean it, there is a specific MAF cleaner you can buy at any auto parts store. Do not use anything to clean it but this cleaner. The platinum "hot" wire in this complex circuit is very fine and can get contaminated very easily. It also gets to 500+ degrees at operating temperature, so the possibility for a fire with other cleaners is great as well. Take out the sensor from the MAF housing via the two torx screws holding the sensor to the housing. Give the wire a quick, thorough blast with the MAF cleaner and set on a clean dry rag to dry for a couple of minutes. Replace. It should now be clean.

Thanks,
Alex
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Monday, March 11th, 2019 AT 5:08 PM
Tiny
NORTHPORTMAZDA
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Getting some of the MAF cleaner tomorrow from a friend.

Incase I have to get a new ECU computer, which number would my 1991 Mazda B2600i 2.6l 4x4 be?

G61818881A
G61818881AR
G61818881R

I can't find anything that will tell me clearly. What's the difference?

Thanks for all you help!
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Monday, March 11th, 2019 AT 11:44 PM
Tiny
SCGRANTURISMO
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Hello again,

Usually there will be a sticker on the front with your vehicle's strategy code on it. Your vehicle will have a strategy code for it, but it is shortened down to the first four letter/number combination. For instance in big writing at top of the sticker it might say G745. That is the strategy code for your vehicle, G745. You will need to replace your ECU with one that has the strategy code G745. This is an example, but look for the sticker on your ECU.

Thanks,
Alex
2CarPros
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Tuesday, March 12th, 2019 AT 2:12 PM
Tiny
NORTHPORTMAZDA
  • MEMBER
  • 14 POSTS
So I finally got a hold of a voltage/ohmmeter. I tested everything in the control system and everything checks out except for the air intake thermal sensor. It's not quite reading within the specs that it calls for so I'm going to replace that and see if it makes any difference. If this is bad how would that affect this issue going on?

The other thing I noticed was when I disconnect the battery and hold the brake pedal down four five seconds and hook the battery backup the vehicle seems to idle and run okay until either 1 you turn the truck off and try to restart it or two it does okay for 10 to 15 minutes and then the problem returns. Would that have something to do with the ECU?

The other question I have is I've been trying to check to see if there is any engine codes. I take the single pin-red wire+green connector and "jump" or ground out to the negative ground on the battery. But I can't get this way of doing it to work. Is there some special way of doing this that I don't know about? Also, when you turn the key on, on the dashboard there is a check engine light, brake light, oil light, battery light, ABS light. The only one that goes out will be the ABS light. All the other lights stay on until you start the vehicle. According to what I've been reading if you jump that single connector and turn the key on if the engine light comes on and then goes off there are no codes, how is this possible to check if my check engine light doesn't go off until you start the vehicle?

I will let you know what I find out when I replace the air intake sensor tomorrow.
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Wednesday, March 13th, 2019 AT 9:58 AM
Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 42,833 POSTS
Please let us know what you find. We are interested to see what it is.
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Sunday, March 17th, 2019 AT 6:59 PM
Tiny
NORTHPORTMAZDA
  • MEMBER
  • 14 POSTS
Though the air intake sensor tested and looked like it needed to be replaced that was not the problem. Even pulled the MAS and cleaned it to no avail.

In researching more on the internet I came across a guy who knows the b2600i inside and out. Has very good videos, here are two great ones:

Most Common Problems:
https://m.youtube.com/watch?v=869H8hD3u7U

On the ECU:
https://m.youtube.com/watch?v=uK8HeIyKiRY

He says this particular model has a problem with the ECU. There is a specific part in it that has a tendency to bleed and when it does it bleeds onto the circuit board and starts corroding it.

Almost anytime you have problems like I'm having this is what's happened to the ECU and it needs to be either fixed, which you can do yourself if you know what you're doing, or replaced.

I pulled the ECU and opened it up and sure enough around that particular unit the board was corroded around it. I have another ECM that will be here Tuesday and I will let you know what happens when I plugged her in a start it up.
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Sunday, March 17th, 2019 AT 7:48 PM
Tiny
SCOTT HANKEMEIER
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Just FYI, 91 Mazda B2600i. Randomly dies, no spark. Sits 10-20min starts up and runs again. Distributor pickup is very common and was the problem. Its the hall or optical sensor in the distributor. Replaced distributor. Common on all the b- series 86-93. Carb models you can get just the pickup or ignitor I think it's called sometimes. Also mine had idle surge/hang at 2k, hard or no start, fuel smell, no power at all, won't Rev up much, flooding out. That was the coolant temp sensor, walker has a sensor with connector. That fixed it.
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Monday, March 18th, 2019 AT 12:27 PM

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